Machine vision systems are commonly used in industry for high speed inspections. In particular, these systems are used to obtain digital images of objects in order to determine, with a computer, whether the object is of "acceptable" quality with respect to predetermined specifications. For example, a system may inspect a semiconductor chip package to determine whether each of the leads of the package have the proper dimensions. A system may also inspect for coplanarity of solder balls on ball grid arrays.
Patterns such as bar codes and data codes are also imaged by such systems. Images of these patterns are analyzed by a computer and in order to "read" the information represented by these codes.
In a machine vision system, an object (or pattern) is typically imaged by illuminating the object with light sources and capturing the light reflected from the object with a video camera (i.e., a photodetector). A digital image is formed from the image received by the camera and the digital data is analyzed by a computer in order to determine characteristics of the object or pattern.
Obtaining a proper contrast between the object or pattern and the background is critical to obtaining an image of sufficient clarity for accurate analysis by a computer. In current practice, an engineer or knowledgeable user obtains the proper contrast by varying the positions of the light sources with respect to the object or pattern being viewed and with respect to the video camera recording the scene. Additionally, the intensity and possibly the polarization and color of the light sources are varied. To achieve the desired contrast, the illumination is often manipulated to make the background either dark with respect to the object features or pattern (dark-field illumination) or bright with respect to the object features or pattern (bright-field illumination). Obtaining the proper illumination is particularly difficult when working with specular (mirror-like) surfaces, especially when the specular surfaces are curved or multifaceted.
One technique for illuminating an object for imaging purposes is described in U.S. Pat. No. 5,461,417 issued to White et al. (the "White '417" patent), expressly incorporated herein by reference. The White '417 patent discloses a system for providing a continuous, uniform, diffuse lighting environment. This system is satisfactory for certain types of applications. Another technique for illumination is described in U.S. Pat. No. 5,187,611 issued to White et al., expressly incorporated herein by reference. In this patent, a Diffuse On-Axis Light (DOAL) is described which is also beneficial in certain applications. However, good contrast sometimes requires edges to be highlighted which is best obtained from collimated unidirectional light, not uniformly diffuse light.
For certain objects, it may be advantageous to sequentially illuminate an object from a number of different viewpoints and take a picture of the object for each illumination. The pictures can then be combined into a single image. Such a system is described in U.S. Pat. No. 5,060,065 issued to Wasserman, expressly incorporated herein by reference. It may be desirable, for example, to image an object using a bright-field illumination method and subsequently image the same object using a dark-field illumination method. The bright-field and dark-field images can then be individually analyzed or can be first combined, and then analyzed.
Unfortunately, the sequential illumination method increases capture time since a separate picture is required for each illumination--each video picture typically requires 1/30 second. Thus, if lights at three different locations from the object are utilized, three pictures would be required.
Furthermore, the combined image tends to look smeared if there is any relative motion between the object and the camera. For example, vibration may cause the object to move slightly. Since an image of the object before the motion and after the motion will not exactly coincide, the combined image will appear smeared.